can clearly see why the
embryos of mammals, birds, reptiles, and fishes should be so closely
alike, and should be so unlike the adult forms. We may cease marvelling
at the embryo of an air-breathing mammal or bird having branchial slits
and arteries running in loops, like those in a fish which has to breathe
the air dissolved in water, by the aid of well-developed branchiae.

Disuse, aided sometimes by natural selection, will often tend to reduce
an organ, when it has become useless by changed habits or under changed
conditions of life; and we can clearly understand on this view the
meaning of rudimentary organs. But disuse and selection will generally
act on each creature, when it has come to maturity and has to play its
full part in the struggle for existence, and will thus have little power
of acting on an organ during early life; hence the organ will not be
much reduced or rendered rudimentary at this early age. The calf, for
instance, has inherited teeth, which never cut through the gums of the
upper jaw, from an early progenitor having well-developed teeth; and we
may believe, that the teeth in the mature animal were reduced, during
successive generations, by disuse or by the tongue and palate having
been fitted by natural selection to browse without their aid; whereas in
the calf, the teeth have been left untouched by selection or disuse,
and on the principle of inheritance at corresponding ages have been
inherited from a remote period to the present day. On the view of each
organic being and each separate organ having been specially created, how
utterly inexplicable it is that parts, like the teeth in the embryonic
calf or like the shrivelled wings under the soldered wing-covers of some
beetles, should thus so frequently bear the plain stamp of inutility!
Nature may be said to have taken pains to reveal, by rudimentary organs
and by homologous structures, her scheme of modification, which it seems
that we wilfully will not understand.

I have now recapitulated the chief facts and considerations which have
thoroughly convinced me that species have changed, and are still slowly
changing by the preservation and accumulation of successive slight
favourable variations. Why, it may be asked, have all the most eminent
living naturalists and geologists rejected this view of the mutability
of species? It cannot be asserted that organic beings in a state of
nature are subject to no variation; it cannot be proved that the